Signals are received and measured for various purposes. Satellite signals or other beacon signals may be measured for example for enabling a positioning of a device receiving the signals.
A positioning of a device is supported for instance by various Global Navigation Satellite Systems (GNSS). These include for example the American Global Positioning System (GPS), the Russian Global Navigation Satellite System (GLONASS), the future European system Galileo, the Space Based Augmentation Systems (SBAS), the Japanese GPS augmentation Quasi-Zenith Satellite System (QZSS), the Locals Area Augmentation Systems (LAAS), and hybrid systems.
The constellation in GPS, for example, consists of more than 20 satellites that orbit the earth. Each of the satellites transmits two carrier signals L1 and L2. One of these carrier signals L1 is employed for carrying a navigation message and code signals of a standard positioning service (SPS). The L1 carrier phase is modulated by each satellite with a different C/A (Coarse Acquisition) code. Thus, different channels are obtained for the transmission by the different satellites. The C/A code is a pseudo random noise (PRN) code, which is spreading the spectrum over a 1 MHz bandwidth. It is repeated every 1023 bits, the epoch of the code being 1 ms. The carrier frequency of the L1 signal is further modulated with navigation information at a bit rate of 50 bit/s. The navigation information comprises inter alia ephemeris and almanac parameters. Ephemeris parameters describe short sections of the orbit of the respective satellite. Based on these ephemeris parameters, an algorithm can estimate the position of the satellite for any time while the satellite is in the respective described section. The almanac parameters are similar, but coarser orbit parameters, which are valid for a longer time than the ephemeris parameters. The navigation information further comprises for example clock models that relate the satellite time to the system time of GPS and the system time to the Coordinated Universal Time (UTC).
A GPS receiver of which the position is to be determined receives the signals transmitted by the currently available satellites, and it detects and tracks the channels used by different satellites based on the different comprised C/A codes. Then, the receiver determines the time of transmission of the code transmitted by each satellite, usually based on data in the decoded navigation messages and on counts of epochs and chips of the C/A codes. The time of transmission and the measured time of arrival of a signal at the receiver allow determining the pseudorange between the satellite and the receiver. The term pseudorange denotes the geometric distance between the satellite and the receiver, which distance is biased by unknown satellite and receiver offsets from the GPS system time. Moreover, pseudorange contains various error terms including troposphere and ionosphere delay as well as multipath.
In one possible solution scheme, the offset between the satellite and system clocks is assumed known and the problem reduces to solving a non-linear set of equations of four unknowns (three receiver position coordinates and the offset between the receiver and GPS system clocks). Therefore, measurements from at least four satellites are required in order to be able to solve the set of equations. The outcome of the process is the receiver position.
Similarly, it is the general idea of GNSS positioning to receive satellite signals at a receiver which is to be positioned, to measure the pseudorange between the receiver and the respective satellite and further the current position of the receiver, making use in addition of estimated positions of the satellites. Usually, a PRN signal which has been used for modulating a carrier signal is evaluated for positioning, as described above for GPS.
GNSS and assisted GNSS are used more and more for location based services which are mostly focusing around personal navigation. GNSS and assisted GNSS technology is integrated to this end for instance in personal navigation devices (PND) and mobile terminals like smart phones.
In a typical implementation, a GNSS receiver measures the satellite signals, calculates the position solution and delivers the position information to the navigation application. In turn, the navigation application estimates the location of the user on the map and on the route and then updates the display accordingly. A good navigation experience requires frequent position and velocity updates from the GNSS receiver. PNDs and navigation enabled smart phones could update the position information for example once a second, which means that also the GNSS receiver needs to operate at that rate.
While measurements of signals have been described by way of example with reference to GNSS based applications, it becomes readily apparent that also various other applications may rely on measurement results that are reported at fixed reporting intervals.